1. Field of the Invention
The present invention is in the field of hydraulic accumulator systems and more specifically, but not limited to, hydraulic hybrid powertrains for motor vehicles.
2. Prior Art
Hydraulic accumulators typically operate over pressure ranges between about 1,000 psi and up to 5,000 psi or greater. Conventional accumulators are composed of a single vessel that accommodates both the charge gas (the "gas side") and the fluid (the "fluid side"). The entire vessel must be strong enough to accommodate the maximum anticipated pressure, although this pressure is only attained when the accumulator is near its full fluid capacity. As fluid is discharged from this maximum pressure state, pressure declines rapidly after only a fraction of the fluid volume has passed out, and throughout the rest of the discharge cycle the pressure is much lower than the maximum pressure. In other words, the single-vessel design of conventional accumulators requires that the entire volume of the vessel be designed to withstand the maximum pressure, rather than only the volume of the working fluid that is actually discharged at that pressure. Over the greater part of its duty cycle, a conventional single-vessel accumulator is essentially overdesigned, increasing its weight and cost.
Having all of the fluid and charge gas contained in a single vessel maximizes the severity of a catastrophic event such as a rupture. If a rupture were to occur in a conventional accumulator, the energy contained in the entire charge of fluid and gas could be released at once in a rapid and uncontrollable manner. A rupture of any portion of the vessel could result in failure of the entire system.
While not a common or widespread practice, accumulator systems have been built by connecting a gas pressure vessel, acting as the gas side, to another pressure vessel that serves as the fluid side. This separates the gas and fluid sides, but does not address the problem of the mismatch between vessel strength and the average actual pressure experienced. Thus it does not lead to significant weight or cost reductions. While the severity of a rupture would be reduced somewhat by having the gas and fluid systems in two separate packages by adding a shut-off valve between the two vessels, the entire gas charge or the entire fluid charge would still be involved in case of a rupture, which could still cause significant damage.